\setbeamertemplate{background}{\tikz[overlay, remember picture, help lines]{
    \foreach \x in {0,...,15} \path (current page.south west) +(\x,6.75) node {\small$\x$};
    \foreach \y in {0,...,7} \path (current page.south west) +(15.5,\y) node {\small$\y$};
    \foreach \x in {0,0.5,...,15.5} \draw (current page.south west) ++(\x,0) -- +(0,6.6);
    \foreach \y in {0,0.5,...,7} \draw (current page.south west) ++(0,\y) -- +(15.5,0);
  }
}

% \setbeamertemplate{background}{\tikz[overlay, remember picture, help lines]{
%     \foreach \x in {0,...,15} \path (current page.south west) +(\x,7.25) node {\small$\x$};
%     \foreach \y in {0,...,7} \path (current page.south west) +(15.5,\y) node {\small$\y$};
%     \foreach \x in {0,0.5,...,15.5} \draw (current page.south west) ++(\x,0) -- +(0,7.25);
%     \foreach \y in {0,0.5,...,7.5} \draw (current page.south west) ++(0,\y) -- +(15.5,0);
%   }
% }